1. Field of the Invention
This invention relates to the synthesis of ammonia. More particularly, this invention relates to a novel continuous process for producing ammonia which provides a more efficient and less costly means of controlling the temperature at which the synthesis reaction is carried out. This in turn provides more efficient heat recovery, using less heat exchanger surface area, and higher conversions of synthesis gas to ammonia than hitherto-practiced ammonia synthesis processes.
2. Description of the Prior Art
Ammonia is produced commercially today by continuous processes which involve the seemingly straightforward reaction between stoichiometric amounts of nitrogen and hydrogen: EQU N.sub.2 +3H.sub.2 .fwdarw.2NH.sub.3.
In practicing such processes, a gaseous mixture containing nitrogen and hydrogen is passed sequentially over two or more catalyst beds containing, for example, finely divided iron or promoted iron catalyst, at relatively high pressure and controlled temperature.
The reaction is exothermic, hence, the equilibrium will be shifted to the right as the reaction temperature is lowered. As a practical matter however the reaction temperature must be maintained at a sufficiently elevated level to permit the synthesis of acceptable quantities of product in a reasonably short time. This is true even though a catalyst is customarily employed to accelerate the reaction rate. Thermodynamic considerations also militate in favor of the ammonia-producing reaction's being carried out at high pressures; collisions between nitrogen and hydrogen gas molecules are necessary to effect the synthesis. Consequently, the process is conventionally carried out at pressures of over 100 atmospheres, although it has been disclosed in the prior art that processes of this general type can be practiced at pressures as low as 20 atmospheres; see, for example, U.S. Pat. Nos. 3,368,869; 4,153,673; 4,163,775; 4,250,057 and 4,271,136.
As can be seen, then, an appropriate balance must be struck between thermodynamic, kinetic and economic considerations when determining the conditions--particularly temperature--at which a commercially attractive ammonia synthesis will be carried out.
A typical prior art process for synthsizing ammonia--for example that disclosed in Wright et al U.S. Pat. No. 3,851,046 or in Grotz U.S. Pat. No. 4,510,123 -- involves:
(a) heating synthesis gas containing nitrogen and hydrogen in roughly stoichiometric amounts to a proper temperature level,
(b) passing this gas over two or more catalyst beds in series, these beds containing, for example, iron or promoted iron catalyst, to produce a reactor effluent which is at a higher temperature than the original synthesis gas mixture due to the exothermic nature of the reaction and which contains some percentage of ammonia, representing for example 15 to 35% of the total volume of the reactor effluent, and
(c) cooling this reactor effluent to recover heat for various uses in the plant and to prepare the effluent for further processing to separate ammonia from unreacted hydrogen, nitrogen, and any inert diluent(s) present.
Ordinarily, the temperature of the gas emerging from the first and any subsequent catalyst beds is sufficiently high to be thermodynamically inhibitory to further ammonia-forming reaction. Therefore, the effluent from one catalyst bed must be cooled if it is to be passed through another catalyst bed to increase the percentage conversion of the synthesis gas to ammonia. Temperature regulation is thus of prime importance to the efficiency of multiple catalyst bed ammonia synthesis processes.
Apparatus in which the Wright et al patent's process can be carried out is illustrated schematically in FIG. 1 attached hereto; bypass valves 102 and 104 in FIG. 1 are not specifically disclosed in the Wright et al patent, but have been included for reasons discussed hereinbelow.
With reference to FIG. 1, fresh syngas introduced through a conduit 106 is passed through a conduit 108 to a heat exchanger 110 and heated therein to a temperature of 280.degree. C. The thus-heated syngas is then passed through a conduit 112 to a second heat exchanger 114 and heated therein to a temperature of 400.degree. C. The high temperature syngas is then passed through a conduit 116 to a catalytic converter 118 in which the exothermic ammonia-forming reaction causes the temperature to rise to about 520.degree. C. The partially converted gas exiting the catalytic converter 118 through a conduit 120 is then cooled in the second heat exchanger 114 to a temperature of about 400.degree. C, then passed through a conduit 122 to a second catalytic converter 124, where again the conversion of hydrogen and nitrogen to ammonia results in a temperature rise in the stream, this time to about 480.degree. C. Effluent from the second catalytic converter 124 passes through a conduit 126 to a steam generator or superheater 128, and is cooled to about 320.degree. C. by generating high pressure steam in the steam generator 128. The thus-cooled stream is then passed through a conduit 130 and further cooled by heat exchange with fresh syngas in the first heat exchanger 110.
Temperature regulation in such a process could be accomplished by use of the added bypass valves 102 and 104. There are, however, limitations to the effectiveness of this means of temperature control. For a given temperature in the conduit 112, if the bypass valve 104 is opened to cause a lower temperature in the conduit 116, the conversion in the first catalytic converter 118 would increase, casing an increased effluent temperature in the gas exiting the first catalytic converter through the conduit 120. Since opening the bypass valve 104 would also result in less cooling of the effluent from the first catalytic converter 118 in the second heat exchanger 114, the temperature of the gas entering the second catalytic converter 124 through the conduit 122 would increase, resulting in a decrease in conversion in the second catalyst bed. Thus, the overall conversion achieved in the first and the second catalyst beds would remain the same as it was before the valve 104 was opened.
To decrease the temperature of the gaseous reactants in the conduit 122 and the second catalytic converter 124, a bypass valve 102 would have to be used. Opening the bypass valve 102 would decrease catalytic converter inlet temperatures, resulting in higher conversions. Higher conversions in turn would permit a lower synthesis pressure, resulting in savings in steam consumption in the turbine that drives the synthesis gas compressor or any other compressor in the plant. These savings would be offset, however, by the loss of steam production from the steam generator as a result of the lower temperature of the gas exiting the catalytic converter 124. Thus, opening the bypass valve 102 would have the disadvantage of reducing overall heat recovery.
U.S. Pat. No. 4,510,123 discloses a three or more catalyst bed ammonia synthesis system in which the temperature of the first bed is regulated by heat exchange between the effluent of the first bed and fresh syngas and the temperatures of subsequent beds are regulated by high pressure steam generation. The use of additional catalyst beds permits higher conversions to ammonia. However, the first two beds of this three or more bed system are subject to the same temperature control limitations as described above for the process of the Wright et al patent.
Some prior art ammonia processes, for example, those described in the booklet "Topsoe S-200 Ammonia Synthesis Process", August, 1985, recover heat from the final catalytic converter bed by first generating high pressure steam and also heating boiler feed water. This method achieves efficient temperature regulation in all the catalyst beds, but has the disadvantages of adding pressure drop in the synthesis loop, adding the capital cost of an additional heat exchanger, and requiring the use of cold boiler feed water, which may not be available.
Another prior art means of controlling temperature during ammonia synthesis is by the use of "quench" type processes, in which effluent from one catalyst bed is mixed with "cold" fresh synthesis gas, thus lowering the temperature of the mixture entering the next catalyst bed to the proper level. While quenching may be repeated for as many beds as desired, obviously not all of the synthesis gas will pass through all of the catalyst beds, and each quench reduces the amount of high pressure steam that can be generated.
U.S. Pat. No. 4,230,680 describes a three bed process for producing ammonia from syngas in which temperature is controlled by passing a portion of the effluent from each of the three catalyst beds through heat exchangers to which all or a portion of the fresh syngas is also fed to provide a heat sink. Effluent from the third bed is cooled by "various plant fluid(s)", "such as boiler feed water". If boiler feed water or other such cooling fluid is used, higher ammonia conversions in the first two beds can be achieved than are achievable in the processes previously discussed. But since no high pressure steam is generated, much less heat recovery is achieved. And, if effluent heat is used to generate steam instead of to heat boiler feed water, ammonia conversions are limited as in the previously discussed processes.
U.S. Pat. No. 4,215,099 to Pinto et al discloses a process for producing ammonia or methanol in which the synthesis gas fed to the first catalyst bed is in heat exchange with a coolant, preferably feed synthesis gas, and the second catalyst bed is adiabatic. This system is said to give a higher conversion to ammonia in the first catalyst bed, but does so with reduced heat recovery.
U.S. Pat. No. 4,213,954 discloses an ammonia synthesis in which steam is superheated in the synthesis section of the process to better control steam rates in the event of shutdown of the synthesis section, i.e., to avoid overheating steam superheaters in the synthesis gas generating section of the process. This process is operated at a synthesis pressure under 150, and preferably 40-80, bar abs, positions the steam superheater so that it will cool reacted gas before this gas is cooled by any other heat exchange, and achieves 15-30% or more of the total plant steam superheating.
The need exists, therefore, for a continuous process for producing ammonia which provides, in an economical manner, efficient temperature control, heat recovery and catalyst utilization with no sacrifice in yield of recoverable product.